Recently, a large variety of synthetic resin materials have been developed and marketed and are being used in ever increasing numbers in many industrial fields. The result is that the quantity of the synthetic resin discarded is increasing, and its disposal has become a serious social problem. If directly incinerated, the discarded resin yields toxic gases or damages an incineration furnace due to significant heat of combustion to impart severe load on the environment.
Among known processing methods for disposal of the wasted resins, there are a method consisting in incinerating the wasted resin after turning the wasted resin into low molecular material by pyrolysis or chemical decomposition, and a method consisting in using the wasted resin for land-filling. However, the processing by incineration is accompanied by emission of carbon dioxide and hence may give rise to global warming. Moreover, if sulfur, nitrogen or halogens are contained in the incinerated resin, toxic emission gases, generated on incineration, may prove a cause of pollution of atmospheric air. On the other hand, in case the resin currently in use is discarded after use and used for land-filling, most of the resin is left without being decomposed for prolonged time, thus causing soil pollution.
In order to cope with this problem, manufacture and usage of plastics, mainly composed of bio-cellulose, derived from natural materials, or starch, natural polyester by fermentation, cellulose based esters of low substitution degree, natural polyesters by fermentation, or aliphatic polyester resins by chemical synthesis, are under investigations as biodegradable plastics. The biodegradable resins are biochemically decomposed by microorganisms into carbon dioxide and water, for example. Thus, even if these biodegradable resins are discarded in the environment, they are readily decomposed into low molecular weight compounds innocuous to environment. Hence, the use of biodegradable resins decreases the adverse effect of disposal on global environment. For these reasons, researches into the use of the biodegradable resins for disposable products, centered about sundries for our everyday life, goods of hygiene or toys, are currently underway.
The state-of-the-art biodegradable resins are satisfactory from the perspective of safety to environment, as discussed above. However, the state-of-the-art biodegradable resins are not satisfactory as regards flame retardant performance, the demand for which is increasing from the viewpoint of safety in putting the resins to actual use. In particular, in the case of electrical products, there is a demand for facilitating post-recovery processing thereof by forming the casing from the biodegradable resin. However, for use as a casing for an electrical product, the material used must comply with the prescriptions for flame retardant properties as provided for in the Japanese Industrial Standard (JIS) or in the UL Standard (Underwriter Laboratory Standard). The biodegradable resins, currently in use, are not up to the prescriptions for flame retardant properties.
Moreover, when the biodegradable resins are applied to, for example, practical articles, such as casings of electronic equipment, the resins are required to exhibit high flame retardant properties and durability under high temperature high humidity conditions. For example, with a portable audio product, for example, it is required that physical properties, such as strength, shall be maintained for three to seven years under the conditions of a temperature of 30° C. and a relative humidity of 80%.
A variety of researches have so far been conducted in order to confer physical properties suited to practical molded products on the biodegradable resins. For example, a method consisting in blending a suitable amount of a biodegradable resin, displaying the properties, similar to those of rubber having a low glass transition temperature, to the aliphatic polyester resins, as typical of the biodegradable polymer, for improving biodegradability and moldability, has been proposed as a first technique. A method consisting in adding calcium carbonate and/or magnesium carbonate to the aliphatic polyester resins, for improving the mechanical strength, has also been proposed as a second technique. In addition, a method consisting in melting poly-3-hydroxy lactic acid, followed by quenching and solidifying to form a molded product having a degree of crystallinity less than 50%, for improving biodegradability, has been proposed as a third technique.
The molded products, formed of the biodegradable resins, so far proposed, are presupposed to be mainly used for films or packaging materials, while sufficient precautions have not been taken as to flame retardant performance or to preservation characteristics.